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Thu, 17 Mar 2022 17:00:09 +0000 Received: from BN0PR11MB5744.namprd11.prod.outlook.com ([fe80::ec9a:f02a:6fc1:c6c]) by BN0PR11MB5744.namprd11.prod.outlook.com ([fe80::ec9a:f02a:6fc1:c6c%6]) with mapi id 15.20.5081.017; Thu, 17 Mar 2022 17:00:08 +0000 Message-ID: <87c00fe2-e4fc-b006-f608-3dc2a209ed77@intel.com> Date: Thu, 17 Mar 2022 10:00:03 -0700 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Firefox/91.0 Thunderbird/91.7.0 Subject: Re: [PATCH v3 19/21] x86/resctrl: Rename and change the units of resctrl_cqm_threshold Content-Language: en-US To: James Morse , , CC: Fenghua Yu , Thomas Gleixner , Ingo Molnar , Borislav Petkov , H Peter Anvin , Babu Moger , , Jamie Iles , "D Scott Phillips OS" , , , References: <20220217182110.7176-1-james.morse@arm.com> <20220217182110.7176-20-james.morse@arm.com> From: Reinette Chatre In-Reply-To: <20220217182110.7176-20-james.morse@arm.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MW4PR03CA0129.namprd03.prod.outlook.com (2603:10b6:303:8c::14) To BN0PR11MB5744.namprd11.prod.outlook.com (2603:10b6:408:166::16) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 95d2a2ea-2ac7-466e-c14c-08da0837972d X-MS-TrafficTypeDiagnostic: BN6PR11MB4066:EE_ X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr X-Microsoft-Antispam-PRVS: X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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The interface > between the architecture's resctrl code and the filesystem ought to > treat everything as bytes. > > Change the unit of resctrl_cqm_threshold to bytes. resctrl_arch_rmid_read() > still returns its value in chunks, so this needs converting to bytes. > As all the callers have been touched, rename the variable to > resctrl_rmid_realloc_threshold, which describes what the value is for. > > Signed-off-by: James Morse ... > @@ -762,10 +763,7 @@ int rdt_get_mon_l3_config(struct rdt_resource *r) > * > * For a 35MB LLC and 56 RMIDs, this is ~1.8% of the LLC. > */ > - resctrl_cqm_threshold = cl_size * 1024 / r->num_rmid; > - > - /* h/w works in units of "boot_cpu_data.x86_cache_occ_scale" */ > - resctrl_cqm_threshold /= hw_res->mon_scale; > + resctrl_rmid_realloc_threshold = cl_size * 1024 / r->num_rmid; > > ret = dom_data_init(r); > if (ret) > diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c > index 7ec089d72ab7..93b3697027df 100644 > --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c > +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c > @@ -1030,10 +1030,7 @@ static int rdt_delay_linear_show(struct kernfs_open_file *of, > static int max_threshold_occ_show(struct kernfs_open_file *of, > struct seq_file *seq, void *v) > { > - struct rdt_resource *r = of->kn->parent->priv; > - struct rdt_hw_resource *hw_res = resctrl_to_arch_res(r); > - > - seq_printf(seq, "%u\n", resctrl_cqm_threshold * hw_res->mon_scale); > + seq_printf(seq, "%u\n", resctrl_rmid_realloc_threshold); > > return 0; > } This change has some user visible impact that I am still digesting but thought that I would share for your consideration. As seen in the above two snippets, the original code did: resctrl_cqm_threshold /= hw_res->mon_scale; /* resctrl_cqm_threshold used internally */ resctrl_cqm_threshold * hw_res->mon_scale; /* this is displayed to user */ The original loss due to truncation during the division is not recovered when the value is displayed to the user the user may see significant differences before and after this patch. I tried this out on a system with a large cache and the before and after information is significant: Before this patch: info/L3_MON/max_threshold_occupancy:147456 After this patch: info/L3_MON/max_threshold_occupancy:196608 As I understand this change indeed represents the information more accurately but I found it noteworthy that this is not just a simple "change the units" and may thus have broader impact and may indeed result in different behavior that should be considered. Reinette